The present invention relates generally to sensing and detecting apparatus, and more specifically to arrangements of sensing elements of varying geometry and position for detecting selected groups of spectral components such as colors, typically used in scanners, electronic cameras, detectors, and the like.
Resolving an electromagnetic signal, for example light, into its constituent wavelengths, for example colors, is well known. Typical apparatus for doing so include prisms, diffraction gratings, thin films, etc., and many applications have been made of the ability to resolve such signals into their constituent parts. Electronic imaging, filtering, and object recognition are several of the more common applications. Electronic imaging applications are of primary concern herein, including those that operate primarily in the visible light region of the electromagnetic spectrum, and those that operate primarily outside that region. (For purposes of the present application "spectral" means both visible and nonvisible regions of the electromagnetic spectrum.) Electronic imaging applications operating primarily in the visible light region include, for example, video cameras, facsimile machines, electronic copiers, etc. Electronic imaging applications operating primarily outside the visible light region include infrared (IR) or ultraviolet (UV) detectors, spectrum analyzers, etc. The aim of these electronic imaging applications in general is to convert an electromagnetic signal (hereafter referred to as a "source image") into a machine manipulable data representation thereof.
Apparatus for producing machine manipulable data representations of a color source image include means for performing at least two functions: first, filtering or resolving the source image and second, detecting selected portions of the resolved source image. Heretofore, these functions have been performed by separate means. For example, U.S. Pat. No. 4,786,964 to Plummer et al. discloses an electronic imaging apparatus including separate filtering means and detector means. Multicolor striped or mosaic optical filters filter all but selected spectral components of the source image. Typically, 3 different color filters are employed to distinguish the primary colors. For an additive process, red, green, and blue are commonly used. For a subtractive process, yellow, green, and cyan are preferred. Although not specified, these filters are typically gelatin filters (such as dye inside a polyimide coating) as known in the art. These filters are placed over a plurality of charge coupled devices (CCDs) which detect the intensity of the light transmitted by each filter.
The general assembly and operation of the apparatus according to Plummer et al. is representative of the state of the art of color electronic imaging. The device of Plummer et al. happens to be a camera, although other references such as U.S. Pat. No. 4,734,760 to Futaki and U.S. Pat. No. 4,580,889 to Hiranuma et al. disclose other applications of similar operation. In a majority of these applications, however, filtering means, as opposed to resolving means, are used to separate the spectral components of the source image. The importance of this distinction is that filtering means reduce the available image intensity as a function of the number of components to be detected, whereas resolving means allow utilization of the maximum image intensity available, regardless of the number of components to be detected.
One variation on the above involves use of multiple light sources of different color to illuminate an object such as a color document. Light will be reflected by the object in regions of similar color to the source, and absorbed otherwise to produce a source image. Sensors such as the above-mentioned CCDs, photodiodes, or the like may then be used to detect the extent of reflection for each light source color, and by additive or subtractive processes the color composition of the object may be approximated.
Another variation on the above general assembly and operation is disclosed in U.S. Pat. No. 4,709,114 to Vincent. A color source image is caused to be incident upon a stack of dichroic plates which are reflective to selected colors and transmissive to all others. Sensors are positioned such that selected reflected color components of the source image, reflected by one plate of the stack, are individually detected. Alignment of the sensors is crucial in this arrangement in order to distinguish the sensing of individual colors.
Yet another variation of the above-described general embodiment is disclosed in U.S. Pat. No. 4,822,998 to Yokota et al. The filtering means disclosed in Yokota et al. comprises a silicon dioxide body formed to have areas of step-wise increasing thickness to define discrete filtering elements which form a set of interference filters. The greater the thickness of the filtering elements, the longer the first-order transmission wavelength. The sensing means disclosed in Yokota et al. are arrays of photodiodes mounted or formed on the surface of a substrate. These photodiodes may be provided with different sensitivities to operate in conjunction with the filtering elements for sensing selected color components. The geometry (i.e., planar aspect) of the photodiodes corresponds to the geometry of the filtering elements. The interference filter is mounted in either touching or spaced apart relationship to the photodiode arrays such that transmission by each element is caused to be incident upon a photodiode.
Each of the devices of the prior art have shortcomings and disadvantages which have been addressed by the present invention. One problem common to all the above-mentioned apparatus is that any compensation for the wavelength dependence of the sensitivity of the material from which the sensors are formed is performed remote of the sensors themselves. That is, compensation is usually performed in processing the output signals from the sensors. This adds complexity to the processing of such signals. It also generally implies added circuitry to implement the compensation.
Related to this, the prior art has heretofore been unable to provide any shaping of the sensitivity curves (i.e., transformations and filtering) of the responses of the sensors other than processing of the output signals of the sensors. Again, this adds complexity and hardware to the processing of such signals.
Furthermore, any filtering performed by the prior art has been by the interposing of physical filtering means between the image source and the detecting means. The result of this limitation is that much of the light intensity of a given wavelength is not delivered to the sensor intended to sense that wavelength; on the contrary, most of the light intensity of a given wavelength is wasted. Transmission filters such as gelatin films filter light by transmitting certain colors of light and absorbing all others. Gelatin film transmission efficiency is at best on the order of 50% in the range of colors they are designed to transmit. Furthermore, in order to filter a color source image into a number of components, say N discrete components (N is commonly referred to as the number of bins the source is divided into), there will be at least N filters. Some portion of the source image must fall on each of the filters (i.e., into each bin). If evenly distributed, there will be at best 1/N times the intensity of the source image falling on each filter. Once filtered, there will be at best 50% of this amount falling on the sensing means. The dichroic filters and interference filters have a much higher transmission efficiency than gelatin filters, however, they must also divide the source image N times (into N bins), where N is the number of components to be detected, thus reducing available image intensity by a factor of N.
In addition, the effect of such filters is generally to pass only a single spectral component or a group of adjacent spectral components (i.e., two or more components of the electromagnetic spectrum whose wavelengths are numerically adjacent one another within the resolution of the filtering and sensing means). It has not been possible to tailor such filters to pass arbitrary selected groups of spectral components for detection.
Another problem not addressed by the prior art is the presently unfilled need for an integral filtering and sensing means. In each of the prior art devices, the filtering elements and the sensing elements are formed separately, then joined. It is therefore desireable to provide an apparatus having sensing means formed directly on, or integrated into, the filtering means.
The realization or discovery of these problems and their solution each form various aspects of the present invention, as described further below.